Micro devices are understood as devices belonging to the general field of Micro Electro Mechanical Systems (MEMS), which includes microelectronics (coils, capacitors, dielectric material), micromechanics (sensors, fast prototyping, biochips), microfluidics (Micro Total Analysis Systems (μTAS) (applications of miniaturized chemical, biochemical and biological systems; micro and nano-scale technologies related to analytical systems, synthesis of compounds, clinical diagnostics, genomics, drug screening, and combinatorial chemistry), micropumps,) and even more.
MEMS industry uses more and more often SU-8 photoresist to define structures on a chip in view of its significant advantages. The SU-8 is a negative, epoxy-type, near-UV photoresist commercially available under this trade name. This photoresist can be as thick as 2 mm and an aspect ratio better than 20 can be achieved with standard contact lithography equipment. That makes it possible to define a significant number of varied structures, which are impossible to realize with other materials. Moreover, it has a very high optical transparency above 360 nm and is thermally stable.
One of the biggest problems of SU-8 photoresist is its adhesion on the layer below. This adhesion depends on the material (of the layer below) but is really affected by the chemical environment. For example, SU-8 delaminates from many surfaces when immersed in KOH (Potassium Hydroxide solution) or TMAH (Tetra Methyl Ammonium Hydroxide solution), whereas it withstands HF (hydrofluoric acid). There are some solutions to improve the adhesion of the SU-8, but none could avoid the lift-off in long KOH or TMAH wet-etching so far.